The electrothermal method for the production of phosphorus involves the reduction of phosphate rock with coke or other carbonaceous reducing agent in the presence of silica in an electric furnace. The produced phosphorus vaporizes and is removed from the furnace as a gaseous products stream, which usually also contains carbon monoxide and appreciable quantities of entrained dust.
The product gas stream often is passed through an electrostatic precipitator to remove the dust, prior to cooling to condense the phosphorus, which is collected under water. The carbon monoxide may be recovered for use as fuel or more usually is flared to atmosphere.
The phosphorus condenser drains into a sump wherein the phosphorus product is collected. Settling of the product may be allowed to occur in the sump or more usually the product is discharged from the sump to a settling and storage tank. Upon settling of the phosphorus-containing product, either in the sump or in a separate vessel, three separate layers generally form.
A layer of relatively high grade phosphorus is obtained at the bottom of the settling vessel. The intermediate layer is a mixture of material, commonly termed "phosphorus mud" or simply "mud" by the applicants, and these terms are used herein to refer to such material. Phosphorus mud consists generally of phosphorus droplets or globules, solid impurities and water, and usually contains about 25 to about 60 wt. % phosphorus. Above the mud layer is a water layer, commonly termed "phossy water" by the applicants. Phossy water consists largely of water but contains some phosphorus and suspended solids. The boundary between the phossy water layer and the mud layer often is not clearly defined.
The bottom phosphorus layer usually is removed from the storage and settling tank to a phosphorus storage tank from where the phosphorus is shipped to customers. The phosphorus mud layer may be pumped to another settling tank where some additional separation of phossy water occurs, this phossy water overflowing back to the primary settling tank. Periodically, typically once-a-day, the phosphorus mud is pumped to large storage tanks where the mud typically has a residence time of several months, enabling further separation of the mud into a lower somewhat impure phosphorus layer ("black" phosphorus), an intermediate mud layer and an upper phossy water layer. The black phosphorus layer can be recirculated to the primary storage and settling tank while the resulting mud is generally an intractable phosphorus-containing emulsion.
Various methods for removing and recovering elemental phosphorus from phosphorus mud have been suggested in the prior art. For example, the applicants distill the mud in an inert atmosphere, usually nitrogen, in the equipment illustrated in U.S. Pat. No. 4,081,333 at high temperature, collect the distilled phosphorus and then dispose of the residual solids as landfill. The disclosure of U.S. Pat. No. 4,081,333 is incorporated herein by reference.
In using the equipment described in the aforesaid U.S. Pat. No. 4,081,333 to remove phosphorus from phosphorus mud to enable the solids to be disposed of safely, the processing takes place in three stages, namely an initial water boil period when the moisture associated with the mud is boiled off, a phosphorus boil period during which yellow (white) phosphorus is boiled off from the solid residue and a tail period in which the solid residue is freed from residual amorphous red phosphorus. The tail period continues until the level of phosphorus in the residue has been decreased to that at which the residue offers no fire hazard or health hazard due to elemental phosphorus.
The quantity of red phosphorus required to be removed is small compared with the yellow phosphorus, estimated typically to be about 10% of the total amount of recoverable phosphorus in the mud, and yet that tail period can form from about 40 to about 70% of the total still batch time. The yellow phosphorus boils at 280.degree. C. at atmospheric pressure and the amorphous phosphorus sublimes at about 400.degree. to 450.degree. C. at atmospheric pressure. Temperatures in the residue reach about 550.degree. C. towards the end of the batch and hence a considerable energy input is required.
The mud processing operation is a batch one and the longer the period of time of the cycle then the lesser is the overall throughput of the still on a long term basis. It is necessary to eliminate the red amorphous phosphorus in order to provide residual solids which are safe for disposal. But the elimination of the relatively small amount of red phosphorus occupies an inordinately large proportion of the cycle time.